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Combustion Analysis Calculator

Chemistry

Mass unit

Worked examples

g

g

g

Add element:

g/mol

Adds the molecular formula.

Result

Empirical formula

C₃H₄O₃

88.06 g/mol

Molecular formula

C₆H₈O₆

n = 2

Molar mass

176.12

g/mol

Degrees of unsaturation

3

DBE

Combustion train

Composition by mass

C 40.92 %

H 4.58 %

O (by difference) 54.50 %

Step-by-step

ElementProductProduct mass (g)FactorElement mass (g)% in sampleMoles per 100 gRatio ÷ smallestSubscript
CCO₂0.74960.272920.204640.923.4071.0003
HH₂O0.20460.111910.022904.584.5431.3344
O (by difference)————54.503.4071.0003

Factors are computed from the atomic masses used across MonoCalc. Ratios were multiplied by 3 to reach whole numbers.

Balanced combustion equation

C₆H₈O₆ + 5 O₂ → 6 CO₂ + 4 H₂O

Idealised textbook convention: N ends up as N₂, S as SO₂ and halogens as HX. Real combustion trains reduce NOₓ to N₂ over hot copper and trap halogens separately.

About This Tool

Combustion Analysis Calculator – From Product Masses to a Formula

Combustion analysis is the classic way to learn what an organic compound is made of. A known mass of sample is burned in excess oxygen, and the products are trapped and weighed. This combustion analysis calculator turns those weights, or the percentages printed by a CHN(S) analyser, into element masses, finds oxygen by difference and returns the empirical formula, plus the molecular formula when you supply a molar mass.

How a combustion train works

The sample burns in a furnace in a stream of O₂. The gas then passes through two absorbers. The first holds a drying agent such as magnesium perchlorate and traps water. The second holds NaOH on a support (Ascarite) and traps carbon dioxide. The order matters: the NaOH absorber would also take up water, so water must be removed first. The mass gain of each tube is the mass of H₂O and CO₂ produced.

Gravimetric factors

All carbon in the sample ends up in CO₂ and all hydrogen in H₂O, so each element's mass is a fixed fraction of the product mass:

  • Carbon: m(C) = m(CO₂) × 12.011 / 44.009 = m(CO₂) × 0.27292
  • Hydrogen: m(H) = m(H₂O) × 2.016 / 18.015 = m(H₂O) × 0.11191
  • Nitrogen as N₂ has a factor of 1; as NH₃ (Kjeldahl) it is 14.007 / 17.031
  • Sulfur as SO₂: 32.06 / 64.058
  • Halogen X as AgX: A(X) / (A(Ag) + A(X)), for example AgCl is 35.45 / 143.32

The calculator computes every factor from the same atomic masses used by the other MonoCalc chemistry tools, so results agree across them.

Oxygen by difference

Oxygen cannot be weighed separately, so it is 100 % − (%C + %H + %N + %S + %X). That means every error in the other elements collects in the oxygen figure. This tool treats a residual within ±0.5 % as experimental error and assumes no oxygen; you can override that with the oxygen-handling control. If the measured elements add up to clearly more than 100 %, no formula is given, because something is wrong with the sample mass or a product mass.

Why N, S and halogens may use another sample

Nitrogen, sulfur and halogens are often measured by separate methods on a second portion of the compound. Because percentages are worked out against the sample that was actually used, results from different portions combine correctly. If you collected nitrogen as a gas volume, convert it to a mass with the Ideal Gas Law Calculator first.

Worked example: vitamin C

Burning 0.5000 g of ascorbic acid gives 0.7496 g CO₂ and 0.2046 g H₂O. Then m(C) = 0.2046 g and m(H) = 0.02290 g, so oxygen by difference is 0.2725 g. The mole ratio C : H : O is 1.000 : 1.334 : 1.000, and multiplying by 3 gives C₃H₄O₃. With a molar mass of 176.12 g/mol the multiplier is 2, so the molecular formula is C₆H₈O₆.

Found versus calculated

Journals often ask authors to show that found percentages agree with those calculated for the proposed formula. A tolerance of ±0.4 percentage points is widely used, although the exact rule differs between journals and some have relaxed it, so check the guidelines of your target journal. The most common reason a real sample misses it is a retained solvent, which is why the optional check lets you add a solvate such as 0.5 H₂O.

Idealised products
The equation assumes nitrogen leaves as N₂, sulfur as SO₂ and halogens as HX. Real instruments reduce nitrogen oxides over hot copper and trap halogens separately.

Related tools

If you already have percentages from another source, use the Empirical Formula Calculator. To check a formula's composition, try the Percent Composition Calculator or the Molar Mass Calculator.

Frequently Asked Questions

Is the Combustion Analysis Calculator free?

Yes, Combustion Analysis Calculator is totally free :)

Can I use the Combustion Analysis Calculator offline?

Yes, you can install the webapp as PWA.

Is it safe to use Combustion Analysis Calculator?

Yes, any data related to Combustion Analysis Calculator only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

How does this combustion analysis calculator work?

It converts the mass of each weighed combustion product into the mass of its element (for example CO₂ to carbon using 12.011 ÷ 44.009), divides by the sample mass to get percentages, finds oxygen by difference, and then turns the percentages into the simplest whole-number ratio of atoms. With a molar mass it also gives the molecular formula.

Why is oxygen found by difference?

Oxygen in the sample cannot be separated from the oxygen in the flowing O₂ supply, so it is not weighed directly. It is taken as 100 % minus the other measured elements. Every error in C, H, N, S and halogen ends up in the oxygen figure, so a small residual (within ±0.5 % here) is treated as experimental error rather than oxygen.

Why are nitrogen, sulfur and halogens sometimes run on a separate sample?

Nitrogen, sulfur and halogens are often determined by different methods (Dumas, Kjeldahl, combustion to SO₂, or precipitation as a silver halide), each needing its own portion of the compound. Tick the separate-sample option for that row and enter its own sample mass; percentages are calculated per determination so they combine correctly.

What does the found-vs-calcd check tell me?

It compares the percentages you measured with those calculated for a proposed formula, optionally including a solvent such as 0.5 H₂O. The ±0.4 percentage-point limit is widely used in journal author guidelines, but requirements differ between journals, so check the one you are submitting to. The tolerance is editable.

What if the elements add up to more than 100 %?

The calculator reports an error instead of a formula. A total above about 100.5 % means the sample mass is too small or a product mass is too large, for example from absorbers that picked up moisture or from a typing slip.

Can I enter nitrogen as a gas volume?

Not directly. This tool takes N₂ as a mass. If you collected nitrogen as a gas volume (Dumas method), convert it to a mass with the Ideal Gas Law Calculator first, then enter that mass here.